1. Field of the Invention
The present invention relates to a radar device that radiates a transmission signal to a target object (hereinafter, referred to as “target”) as an electromagnetic wave, receives a reflected signal resulting from reflecting the transmission signal by the target, and calculates target information based on the reflected signal.
2. Description of the Related Art
Up to now, there has been known a frequency-modulation radar device which calculates a distance to the target and a relative velocity to the target based on peak frequencies of beat signals obtained by transmitting the transmission signals whose frequencies have been continuously modulated to the target, receiving the reflected signals that have been reflected by the target, and mixing the transmission signal and the reflected signal together.
In the radar device of this type, a distance R to the target and a relative velocity V to the target are calculated based on the principle of a general frequency modulation continuous wave (FM-CM) radar as follows.
First, the transmission signal is subjected to the frequency modulation of a triangle wave having a repetition frequency fm and a modulation width ΔT. As a result, the respective frequencies of the transmission signal and the reflected signal are changed with time. It is assumed that a peak frequency of the beat signals in an ascending section where the frequencies of the transmission signal and the reflected signal are linearly increased with time is fu, and a peak frequency of the beat signal in a descending section where the frequencies of the transmission signal and the reflected signal are linearly decreased with time is fd. In this case, those respective peak frequencies are represented by the following Expressions (1) and (2).fu=fr−fp  (1)fd=fr+fp  (2)
In Expressions (1) and (2), fr is an amount proportional to the distance R to the target, and fp is an amount proportional to the relative velocity V (an approaching direction is +) to the target, which are represented by the following Expressions (3) and (4), respectively. In the following Expressions (3) and (4), C represents a light speed, and f0 represents a center frequency of the transmission signal.fr=(4·fm·ΔF/C)·R  (3)fp=(2·f0/C)·V  (4)
Hence, the distance R to the target and the relative velocity V to the target are represented by the following Expressions (7) and (8), respectively, by using addition and subtraction of the peak frequencies fu and fd represented by the following Expressions (5) and (6), from Expressions (1) to (4).fu+fd=2fr  (5)fu−fd=−2fp  (6)R=M1·fr=M1·(fu+fd)/2  (7)V=M2·fp=M2·(−fu+fd)/2  (8)
The following Expressions (9) to (12) are satisfied.M1=C/(4·fm·ΔF)  (9)M2=C/(2·f0)  (10)fu=R/M1−V/M2  (11)fd=R/M1+V/M2  (12)
When the peak frequency fu of the beat signal in the ascending section and the peak frequency fd of the beat signal in the descending section are found, the distance R to the target and the relative velocity V to the target can be calculated by using Expressions (7) and (8).
In the above-mentioned radar device, when a single target exists, the distance R to the target and the relative velocity V to the target can be precisely calculated. However, when a plurality of targets exist, for example, in a situation of detecting other vehicles on a road, it is difficult to precisely calculate the distances to the respective targets and the relative velocities to the respective targets.
Hereinafter, referring to an example in which two targets a and b exist, processing for calculating the distances to the respective targets and the relative velocities to the respective targets is described. In the description, suffix a is attached to values for the target a, and suffix b is attached to values for the target b.
First, distances Ra and Rb to the targets a and b and relative velocities Va and Vb to the targets a and b are calculated based on peak frequencies fua and fub of the beat signals in the ascending section and peak frequencies fda and fdb of the beat signals in the descending section. To achieve this, there is a need to detect pairs of peak frequencies of the beat signals in the ascending section and the descending section of the respective targets a and b, and substitute the pair into Expressions (7) and (8).
However, when two targets exist, there are two combinations as the pairs of the peak frequencies of the beat signals in the ascending section and the descending section of the targets a and b, that is, a correct combination of a pair {fua, fda} with a pair {fub, fdb} and an incorrect combination of a pair {fua, fdb} with a pair of {fub, fda}.
In this example, when the correct combination of the pair {fua, fda} with the pair {fub, fdb} is selected, the distance Ra to the target a and the relative velocity Va to the target a as well as the distance Rb to the target b and the relative velocity Vb to the target b are precisely calculated.
On the contrary, when the incorrect combination of the pair {fua, fdb} with the pair {fub, fda} is selected, values to be calculated are different from the actual distances Ra and Rb to the targets a and b and the actual relative velocities Va and Vb to the targets a and b. In this way, when there is a plurality of targets, because a plurality of peak frequencies occur, the possibility that the incorrect pair is generated (mispairing occurs) is high.
Under the circumstances, in order to prevent the mispairing from occurring, there has been known a radar device, which utilizes such a property that even when the repetition frequency fm, the modulation width ΔF, or the center frequency f0 of the transmission signal (hereinafter, referred to as “modulated signal”) is changed, values of the distance to the target and the relative velocity to the target are not changed as long as the distance and the relative velocity are calculated based on the reflected signal that has been reflected by the same target.
That is, in the radar device, one modulated signal before change in the repetition frequency fm, the modulated width ΔF, or the center frequency f0, and another modulation signal after change, are first transmitted to the same target. Subsequently, the distances to the target and the relative velocities to the target are calculated based on the pair {fu1, fd1} of the peak frequencies of the beat signals in the ascending section and the descending section of the modulated signal before change, and the pair {fu2, fd2} of the peak frequencies of the beat signals in the ascending section and the descending section of the modulated signal after change.
Then, the distance R1 and the relative velocity V1 which are calculated based on the modulated signal before change are compared with the distance R2 and the relative velocity V2 which are calculated based on the modulated signal after change. As a result, when each of the distances and the relative velocities are the same value, it is determined that the correct pairing has been executed, and the calculated distance to the target and the calculated relative velocity to the target are decided (for example, refer to Japanese Patent Application Laid-open No. 2002-236170).
However, the related art suffers from the following problems.
That is, in the radar device disclosed in Japanese Patent Application Laid-open No. 2002-236170, as described above, the distance to the target and the relative velocity to the target are decided based on the pairs {fu1, fd1} and {fu2, fd2} of the peak frequencies of the beat signals in the ascending section and the descending section of the modulated signals before and after changed.
For that reason, when at least one peak frequency cannot be extracted from the peak frequencies of the beat signals, the pair of the peak frequencies cannot be generated. This leads to such a problem that the distances to the target and the relative velocities to the target cannot be obtained.
It is conceivable that at least one peak frequency cannot be extracted in a case where a noise generated in a transceiver or an A/D converter, an offset voltage caused by a device variation or a temperature characteristic, coupling between a transmitter antenna and a receiver antenna, a reflected signal from a point-blank range due to a radome, a reflected signal from another target, a multipath signal, or the like is superimposed on the reflected signal reflected by a desired target, and even if the beat signals obtained from this reception signal are analyzed in frequency, the frequency is reflected to a portion in a given range and cannot be extracted as the peak frequencies.